1. Field of the Invention
This invention relates to industrial-sized briquetting machines, and more particularly to machines utilizing rotating briquetting press rolls which form briquettes in cooperating cavities on the peripheral surfaces of the rolls.
2. Description of the Prior Art
A briquetting press produces briquettes by compressing particulate material between two cooperating rolls, the peripheral surfaces of which contain cavities into which the particulate material is compressed. In the conventional industrial-sized briquetting machine, two rolls of equal diameter are rotated in opposite directions at the same angular velocities. Cavities in the peripheral surfaces of the rolls complement each other at the point of tangency of the two rolls. Particulate material is fed into these cavities between the rolls and compressed to form briquettes. One type of briquetting press may be seen in my U.S. Pat. No. 4,028,035.
Industrial-sized briquetting presses are frequently used to form briquettes of substantially isotropic materials, such as powdered metals and powdered coal. At high pressure, however, the elastic properties of these materials assert themselves. The symetrical design of the conventional briquetting press rolls stores a great deal of elastic energy which, when combined with frictional forces, may split the resulting briquettes into two halves. This effect is commonly referred to as "clam shelling." Furthermore, briquetting rolls of conventional design tend to impose hydrostatic forces on the material being compressed, and isotropic materials subjected to such hydrostatic forces resist plastic flow. Accordingly, much of the elastic energy of conventional roll-type briquetting machines is wasted by the hydrostatic nature of the pressure in the roll cavities.
It is an imbalance in stress, such as a shear stress, which causes plastic flow in isotropic materials. Of course, if the shear stress exceeds a certain critical value, the materials will fracture; this is particularly the case with crystalline materials. If the shear stress is kept below the critical value, the material will be plastically deformed and produce a briquette of greater density and strength than would have been produced by equivalent hydrostatic pressures.
To date, the difficulty has been to design a briquetting press which incorporates the imposition of high shear stresses into a practical mechanical arrangement. The instant invention has achieved this end by providing briquetting press rolls which are rotated in tangential relationship by an associated roll drive mechanism such that the tangential velocities of the roll are different at the point of tangency, while providing registration and cooperation between the cavities on the tangentially adjacent rolls.